This invention relates to fluoroscopic X-ray apparatus and, in particular, to a method and means for maintaining the brightness of a displayed X-ray image constant.
In present day fluoroscopic apparatus, the beam from an X-ray tube is projected through the body and the emergent X-ray image is incident on the input phosphor of an X-ray image intensifier. The intensifier converts the X-ray image to a bright minified optical image which is received by a video camera that effects display of the image on the screen of a video monitor. During examination procedures, the X-ray beam field is usually defined with orthogonally movable pairs of collimator blades or shutters. Usually, when the X-ray tube is energized and the image first appears on the screen it will be necessary to resort to remote controlled adjustment of the collimator blades so that little more than the particular anatomical region-of-interest is exposed to the X-ray beam. This is necessary to minimize X-ray dosage to the patient and is especially important in fluoroscopic examinations which are usually of substantial duration.
It is beneficial to the diagnostician if the displayed X-ray image is held at constant average brightness or intensity during a fluoroscopic examination. It is also desirable to provide for permitting selection of the brightness level that is most comfortable to view and that maximizes the possibility of accurate diagnosis. Hence, an automatic brightness control (ABC) device is customarily provided in the fluorograph systems. Its intended purpose is to maintain average image brightness and account for all variables in the system that might affect brightness.
One ABC method involves detecting the average level of the output signal from the video camera. According to this method, the video signal is integrated by suitable circuitry and the result is compared to a stable reference signal. The error signal resulting from the comparison is then used to drive the X-ray tube current regulator to increase or decrease the intensity of the radiation emitted by the tube until the error signal is nulled. If the X-ray field of view is fixed, it is possible in such system to calibrate the integrator to obtain a preferred brightness level. If the X-ray field is changing, as when the shutters are moved across the beam, the integral of the video signal will be erroneous. To compensate for the error, it has been customary to develop a feedback signal indicative of the shutter or collimator blade opening area. Then, by dividing the integrated brightness signal by the shutter opening area signal, a signal representative of the brightness per unit area was developed for regulating the X-ray tube current. An undesirable feature of total image brightness averaging is that image contrast becomes distorted. Any error in the calculation of the shutter opening area also produces errors in the compensation function. Ordinary image intensity averaging also leads to excessive brightness when an object such as a part of a heavily X-ray attenuating lead glove worn by the radiologist gets into the image field. In addition, X-ray collimator systems usually have straight blades that define X-ray fields that are square in cross-section at all levels along the diverging X-ray beam and this square field or image is incident on an image intensifier input phosphor that is circular. The shutters on the video camera are also usually circular so the image displayed on the video monitor screen is circular. Thus, for example, if the heart is the primary region of interest, it will appear in the circular field but there might be more or less penetrable lung tissue or bone appearing in the field around the heart. Shutter opening area compensation is, therefore, complicated by the different geometries.
Another method involves use of peak brightness detectors. In some images, extreme highlights or lowlights occur which desirably should not be factored into determining the average image brightness. Images were often too dark in the region-of-interest because of the peak detector poor response to highlights.